If your DSi LL firmware is V1.44 or below, Acekard 2i is a good choice too,but it is V1.4.5, R4i Gold 3DS is the best choice now,
because nobody know when will DStwo release

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This is correct, the best card out right now that's readily available and plays every DS game to date. Even class 4 microSD cards play ROMs 100% fine without any lagging or slowdown of any kind. I use a class 4 8GB Sandisk in one, and a class 6 16GB in the newer RTS one for my 2DS.

I of course use my own Pokemon theme. I'll upload the theme if you want, it's quite nice. Has a modern flat Pokemon look to it, and is very polished. I put a lot of testing, time, and care into it to make sure it functions as desired, using the Zelda theme as a template.